Apparatus for the measurement of the temperature of a plastified plastic material at the exit of an extruder

ABSTRACT

An apparatus for the measurement of the temperature of a plastified plastic material at the exit of an extruder, characterised in that the function of the sound, velocity in dependence of the temperature is measured and memorised for at least one plastified plastic material, the sound velocity is measured during the extrusion of the plastic material, and the respective temperature is determined from the velocity measurement values and the function.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

When cables or the like are sheathed with a plastic material, extrudersare used, which plastify the plastic material and inject it round abouta core (conductor). The plastic material is made soft by pressure andheat in the extruder, so that it can be applied to the core withoutproblems and without inclusions. For this purpose, it is desired todetermine the temperature in the plastified material, in order toachieve an optimum viscosity of the melt for the extrusion process.Furthermore, burning or premature crosslinking in the extruder head mustbe avoided. Instead, the crosslinking process has to take placeimmediately after the extrusion. Thus, a reliable temperaturemeasurement optimises the production process.

In such extrusion processes, it is known to measure the temperature ofthe material at different delivery speeds outside of the extruder headwith the aid of a thermocouple. This measurement takes place at adelimited location, and of course it has a relatively long responsetime. The thermocouple measures only the temperature in the outer regionof the melt. If however the thermocouple projects into the melt, thedetermination of the temperature is distorted by sliding frictioneffects. Besides to that, the thermocouple negatively affects the flowproperties of the melt.

It is also known to measure the temperature of the melt with the aid ofan infrared thermometer. However, by doing so, the surface temperatureof the melt can be acquired only up to a depth of a few millimeters.When there are special fillers in the melt, the measurement depth isreduced even further.

However, a capture of the mass temperature of plastics melts that is asaccurate as possible can help to increase the production output of anextrusion plant significantly, namely in that amongst others the idletime (for instance cleaning of the plant, repair etc.) of the plant isreduced by selecting an optimum temperature.

Thus, the present invention is based on the objective to indicate amethod for the measurement of the temperature of at least one plastifiedplastic material at the exit of an extruder, which permits an accuratenon-contact, non-invasive temperature measurement of a melt in a simplerway.

BRIEF SUMMARY OF THE INVENTION

In the method of the present invention, the function of the soundvelocity in dependence of the temperature is measured and memorised forat least one plastified plastic material. During the extrusion of theplastic material, the sound velocity is measured in the extruder,preferably transversely to the flow direction of the plastic material,and the respective temperature is determined from the velocitymeasurement values and the memorised function.

The present invention starts from the finding that the sound velocity inplastics melts, in polyethylene melts in particular, exhibits a strongtemperature dependence. Using this dependence, the mean temperature ofthe plastics melt can be determined across a pipe cross section bydetermining the mean sound velocity.

The ultrasound sensors used in this do not need any contact with theplastics melt, and thus they permit a non-contact, non-invasivedetermination of the mean sound velocity, and by this of the mean masstemperature of the melt.

The temperature determination according to the present invention can beperformed in real time, is accurate and does not impair the melt. As thesound velocity is measured transversely across the melt, it is possibleto determine the mean mass temperature of the melt accurately in thisway.

An additional thermocouple projects directly up to the melt, and so itmeasures the surface temperature of the melt in the edge region of thepipe cross section. When the flow profile of the melt is known,conclusions about the distribution of the temperature within the pipecross section can be drawn from the surface temperature and the meanmass temperature.

The sound velocity in the melt depends also of the pressure in a certaindegree, so that a pressure compensation should occur. According to thepresent invention, the function of the sound velocity in dependence ofthe pressure is measured for at least one plastified plastic materialfor this purpose. The function is memorised, and a pressure measurementtakes place during the extrusion, besides to the sound velocitymeasurement. Even in this, it is of course proceeded in a non-invasivemanner. A pressure-compensated temperature of the melt is thendetermined from the measurement values and the memorised functions ofsound velocity and pressure.

Before an extruder can start with the production, it is of coursenecessary to melt the starting material sufficiently, so that it leavesthe extruder head as a homogeneous extrudate. As is well known, thistakes place by heating the material in the extruder, a furtherwarming-up through friction taking place in this during the advancing inthe extruder with the aid of an extruder screw. By determining thetemperature of the plastics melt and producing extruded samples atcertain time intervals, the machine operator determines when theproduction of an extrudate can be started. A prerequisite is that thematerial is completely molten and there are no more islands of notmolten materials therein, which would impair the quality of the producedmaterial. Consequently, the machine operator observes the material thatwas extruded as a sample at first, and decides based on her/hisexperience when the production can be started.

For reasons of the optimisation of the production output, one desires tokeep the temperature of the plastics melt in the extruder as high aspossible, namely shortly below that temperature at which burning orcarbonization of the material might occur. Upon longer operation of anextruder in particular, it may happen that particles of crosslinked orburned material are formed by which the produced extrudate becomesdefective. Such particles can impair the breakdown resistance in highvoltage cables. In water- or gas pipes, the tightness of the pipe wallcan be impaired.

Therefore, the present invention is based on the further objective toprovide a method for the operation of an extruder for extruding aplastic material in order to form an extrudate of plastic material inwhich the production of the extruder is controlled more accurately andthe formation of extrusion products that are free of defects is ensured.

In the method according to claim 4, the propagation velocity of soundwaves that are sent transversely to the flow direction of the flowablematerial is measured closely to or on the head of the extruder in shorttime distances or continuously during the initial extrusion. A plasticsextrudate is produced only then when the course of the measurementvalues has a substantially steady tendency or is substantially constant.

The present invention starts from the finding that the propagationvelocity of sound waves in the material that is to be extruded does notonly depend on the temperature thereof, but also from the condition inwhich the same is. The propagation velocity in the solely liquidcondition of the plastics material is measurably smaller than that inthe material which is in the solid or only incompletely plastifiedcondition. Therefore, if there are particles or islands of not alreadymolten material in the melt when the extruder operation is started, thisbecomes noticeable through fluctuations of the measurement values forthe propagation velocity of the sound waves. Namely, if the plasticsmaterial would uniformly pass over into melt in the gradual temperatureincrease, the temperature would rise gradually, and the sound velocitywould fall off gradually. The measurement values would have a steadytendency. However, if solid particles or islands migrate through themeasurement region, measurable fluctuations of the propagation velocityoccur. From this it is recognised that the melt in or on the extruderhead has not yet the sufficient homogeneity for beginning theproduction. The other way, the production can be started as soon as thecourse of the measurement values has a steady tendency, for instancewhen it is slightly dropping or constant. Thus, by the method of thepresent invention the machine operator is given a means at hand todetermine the point in time of production by way of the extruder at sucha point in time where products free of defects can be produced as soonas possible, without that unusable material would be unnecessarilyturned out. According to experience, the same is later recycled and isnot lost, but it cannot be used for the intended production for the timebeing.

The course of the measurement values can be displayed by suitable means.

In the method according to claim 5, the propagation velocity of soundwaves is measured continuously or in short time distances during theproduction of the plastics extrudate. The production is stopped when thefluctuation range of the measurement values exceeds a given value. Evenin this method, the finding is used that particles or solid constituentsin a melt influence the propagation velocity of sound waves. As wasalready set forth, local burning and carbonizations of the plasticmaterial due to local overheating can occur during the production. Ifthe same are incorporated into the extruded product, it becomesdefective. This may be of little importance for relatively simpleextrudate-formed plastics products. But for the production of highvoltage cables, of gas or water pipes and the like, such defects cannotbe tolerated. With the aid of the method of the present invention, it ispossible to recognise these defects instantly and to stop the productionprocess in order to maintain the desired quality of the extrudedproducts.

As already set forth, the propagation velocity of sound waves in aplastics melt permits conclusions about the temperature thereof, if itis known how the sound propagation velocity in dependence of thetemperature behaves for special plastics. This can be easily determinedby previous measurements. The methods according to the present inventiondescribed at last can be linked with the temperature measurement, sothat not the course of the sound propagation values is evaluated, butinstead the course of the temperature values.

A device for performing the method of the present invention provides atubular adapter piece at the exit of the extruder, on which anultrasound sender and diametrically to it an ultrasound receiver areradially arranged. The sensors are preferably arranged in a radialrecess in the wall of the adapter piece. If the adapter piece has a wallthickness of 10 mm for instance, a remaining wall thickness of forinstance only 1 mm remains due to the recess. Because the propagationvelocity of sound in steel is almost four times that in the plasticsmelt, the corruption of the measured sound velocity by the remainingwall thickness of the recess plays a minor role. Besides to this, it isof course possible to correct this error. In order to minimise possiblereflections and diffractions at unevennesses, one embodiment of thepresent invention provides that the contact surface in the bottom of therecess is polished.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention will be explained in more detail below by means ofdrawings.

FIG. 1 shows an adapter piece according to the present invention.

FIG. 2 shows a section through the adapter piece according to FIG. 1.

FIG. 3 shows a schematic view of the measuring device 50.

DETAILED DESCRIPTION OF THE INVENTION

While this invention may be embodied in many different forms, there aredescribed in detail herein a specific preferred embodiment of theinvention. This description is an exemplification of the principles ofthe invention and is not intended to limit the invention to theparticular embodiment illustrated

A tubular adapter piece 14 is arranged between the front end of anextruder 10 and the extruder head 12. The extruder and also the fixationof the adapter piece will not be described in detail. The fixation ofthe adapter piece can be made in a simple way by known means. Theextrusion temperature for polyethylene is to be designated by theindicated temperature values of 125° C. and 140° C., respectively. LDPEvaries in its temperature between 100° C. and 200° C.

The adapter piece 14 has a cylindrical wall 16, with a thickness of forexample 10 mm. Diametrically opposite recesses 18 and 20, respectively,are machined from the exterior into the cylindrical wall 16, into whichprobes 22 and 24, respectively, are set in. The probe 22 contains anultrasound sender and the probe 24 an ultrasound receiver. The recessesspare out a remaining wall thickness 26 of for instance 1-2 mm. Thebottom of the recesses 18, 20 is polished, so that no reflections andunnecessary diffractions of the sound signal can take place. With theaid of the ultrasound sender and -receiver, the sound velocity in aplastics melt 30 is measured within the adapter piece 14. For instance,the sound velocity of the polyethylene melt is 1500 m/s. The soundvelocity in steel, from which the adapter piece 14 is made, is about5900 m/s. Therefore, the measurement of the sound velocity yields acertain error that is provoked by the remaining wall thickness 26.However, this plays a minor role in the measurement of the mean soundvelocity of the melt, and if necessary it can be corrected.

Before a temperature measurement of the plastics melt, the dependence ofthe sound velocity from the temperature of the plastics is determinedfor the respective plastic material, which will mostly be linear in therelevant temperature range. This function is memorised in an analysingdevice. By measuring the respective sound velocity, the respectivetemperature can therefore be determined from the function. Thetemperature measurement takes place in real time, and therefore measurescan be initiated immediately, in order to effect a temperature change incase that the measured temperature deviates from a given value.

The sound velocity on its part depends of the pressure in the melt. Inorder to permit a temperature measurement with the aid of the measuredfunction, the pressure in the melt should be also taken into account. Apressure sensor 32 is associated to the adapter piece 14 for thispurpose.

The surface temperature of the melt 30 can also be determined with theaid of a thermocouple 34, which is arranged in the wall 16 diametricallyopposite to the pressure sensor 32, in order to be able to drawconclusions regarding the temperature distribution within the pipe crosssection when the flow profile and the mean mass temperature of the meltare known.

The measurement of the propagation velocity of sound or through thisalso of the temperature of the melt, respectively, can not only be usedin order to produce as near as possible to the upper still admissibletemperature limit in order to keep the output at maximum, but even inorder to control the beginning of an extrusion or to stop anerror-impaired production. A production should begin only when theplastics melt in or on the extruder head, respectively, is homogeneousin a high degree. As an inhomogeneous plastics melt results in anotherpropagation velocity of the sound than a homogeneous one, the selectedtemperature course of the melt is an indication whether homogeneity hasbeen achieved. Before this, solid particles present in the melt wouldcause temperature fluctuations, which should normally not occur. As soonas the temperature course has a steady tendency, the machine operatorcan allow the extruder to produce. This method has the advantage thatthe machine operator must no more take samples, but has only to waituntil the material in or on the extruder head is completely molten.

If there are fluctuations of the temperature course during a production,which cannot occur due to other reasons, this is a sign for aninhomogeneity of the material, due to burning products in particular.The machine operator can therefore stop the extruder when this occurs,and by doing so he/she can prevent the production of defective products.It is to be understood that stopping the extruder or ending theproduction can take place also automatically, in that an analysingdevice detects the fluctuation range of the temperature measurementvalues and provides a stopping signal when the latter exceeds a givenvalue.

1. A device for the measurement of the temperature of a plastified plastic material at the exit of an extruder, comprising: a tubular adapter piece (14) is arranged at the exit of an extruder, on which an ultrasound sender (22) and diametrically to it an ultrasound receiver (24) are radially arranged, a pressure sensor (32) is arranged in the wall (16) and an analyzing device is operatively connected to the ultrasound sender and ultrasound receiver, and the analyzing device is constructed and arranged to measure and record the sound velocity in dependence of the temperature for at least one plastified plastic material, the sound velocity being measured during the extrusion of the plastic material, and the respective temperature being determined from the velocity measurement values and a substantially linear function relating the velocity and temperature in the plastic material, wherein the analyzing device is further constructed and arranged to measure the sound velocity of the plastic material in dependence of the pressure of the plastic material, and further measure a pressure value of the plastic material during the extrusion of the plastic material, and perform a pressure compensation of the temperature measurement with the aid of the pressure measurement value.
 2. A device according to claim 1, characterised in that sender and receiver for ultrasound (22, 24) are arranged in radial recesses (18, 20) in the wall (16) of the adapter piece (14).
 3. A device according to claim 2, characterised in that the remaining wall thickness of the pipe wall is minimized with the aid of the recess (18, 20) such that only a small part of the pipe wall of the adapter oscillates, in order to decrease the blur of the signal in the region of the ultrasound coupling.
 4. A device according to claim 2, characterised in that with the aid of the recess (18, 20), the remaining wall thickness is minimized such with respect to the pipe wall that a lateral signal propagation into the pipe wall is suppressed in a high degree, and a good signal distribution is achieved in the irradiation direction of the ultrasound sender.
 5. A device according to claim 2, characterised in that the bottom of the recess (18, 20) is polished in order to minimize reflections and diffractions of the ultrasound signal on the contact surface.
 6. A device according to claim 1, characterised in that a thermocouple for measuring the surface temperature of the melt is arranged in the wall (16) of the adapter piece (14). 